Signal integrating system



July 1958 D. E. sUNsTElN ET AL 841,70

v SIGNAL INTEGRATING SYSTEM Filed April 9, 1952 2 Sheets-sheet F79. /A MJuly 1, 1958 D. E. sUNsTElN ET AL 2,841,704

SIGNAL INTEGRATING SYSTEM F76. l F76. m ff@ 20), l 70% LXW as /0 CPI fSIGNAL INTEGRATING SYSTEM David E. Sunstein, Bala-Cynwyd, WilliamUssler, Jr.,

Philadelphia, and Allen C. Munster, Hatboro, Pa., assignors to PhilcoCorporation, Philadelphia, Pa., a corporation of Pennsylvania iApplication April 9, 1952, Serial No. 281,414

21 Claims. (Cl. Z50-27) This invention relates to signal integratingsystems and more particularly to systems for integrating a signal conytaining periodically recurring pulse series.

In a radar system, the video signal resulting from target reflectedechoes consists of series of pulses which recur periodically at therepetition frequency of the radar system. Superimposed on this series oftarget reected echo pulses is a random signal resulting from thecombined effects of ground or sea clutter and noise signals generatedwithin the receiver of the radar system. This random clutter signalmakes it dicult if not impossible to detect small targets, for example asubmarine snorkel, under normal or rough sea surface conditions by theuse of non-integrating radar receivers. l

Integrator circuits, commonly called sweep integrators, have beendeveloped for improving the signal-tonoise and the signal-to-clutterratio of a radar system. Basically, a sweep integrator consists of amemory circuit, such as an ultrasonic delay line, with a controlledfeedback circuit coupling the. output of the memory circuit to the inputthereof to form a recirculating signal loop. The delay time of thememory circuit is chosen to be equal to the radar pulse repetitionperiod. Radar video signals are introduced into the recirculating signalloop where they are combined with previously introduced radar videosignals. An output signal proportional in amplitude to a selectedcharacteristic of the combined signal is derived from the recirculatingloop.

Sweep integrators operate on the principle that the frequencydistribution of a series of pulses representing target echoes is a linespectrum with the lines located at harmonics of the pulse repetitionfrequency whereas the spectrum of random noise and clutter signals ismore or less uniformly distributed over the entire Video pass-band. Y

Periodic signals representing echoes from targets -add linearly in therecirculating loop while aperiodic signals, resulting from clutter andthermal noise, add in the more gradual root mean square manner. As aresult, the signalto-noise and/ or clutter ratio in the recirculatingloop is greater than the corresponding ratio at the input to therecirculating loop. The improvement of the signal-tonoise and/ orclutter ratio is a function of the number of successive target signalsstored as a sum in the recirculating loop. In a typical application ofthe sweep integrator it may be desirable to store the sum of as many asto 100 successive target signals within the recirculating loop.successive target signals (based on the criterion of peak signal to R.M. S. noise) the lfeedback factor must be equal to .98 or .99. Thefeedback factor will approach the value 1 as the number of signals to bestored increases but will remain less than one for the storage of afinite number of signals.

The nature of an ultrasonic delay line is such that it is moreconvenient to delay a carrier signal modulated by video intelligencethan it is to delay the video signal directly. For this reason previoussweep integrator circuits employed an amplitude-modulated carrier forstoring In order to achieve optimum storage of 50 to 100 2,841,704Patented July. `1, v195.8

rintelligence within the recirculating loop. A typicalamplitude-modulated sweep integrator circuit includes a delay line fordelaying the amplitude-modulated carrier, an amplifier for amplifyingthe signal coming from the delay line, an amplitude-modulation detector,a signal adding circuit for adding present radar video signals to theoutput of the amplitude-modulation detector, an oscillator circuit, anamplitude-modulator circuit for modulating the signal from theoscillator with the output of the adding circuit and a driver amplifiercoupling the output of the modulator circuit to the input of the delayline. The output of such an amplitude-modulated sweep integrator istaken from the output of the adding circuit. The modulation and thedetection processes which take place every time the signal passes aroundthe loop introduce distortion into the recirculating signal. Also,nonuniformity of frequency response in the amplifiers or in the delayline introduces distortion into the recirculating signal and tends tocause instability in the over-all operating characteristics of the sweepintegrator. In an amplitude-modulated sweep integrator system, anydistortions of the video signal or any amplitude nonlinearities whichoccur anywhere around the feedback loop, are exaggerated by the factorwhere F is the feedback factor of the recirculating loop. In asweepintegrator circuit having a gain of .95 the nonlinearitiesin thefeedback loop would be exaggerated by a factor of 20. If the feedbackfactor is increased to .98 the nonlinearities in the feedback loop areexaggerated by a factor of 50. As a result, it has been found that thehighest feedback factor which can be obtained in a practicalatnplitude-modulation sweep integrator is in thevicinity of .095. Asweep integrator having a feedback factor ofl .95 will store only 20repetition periods.

It isan object of the present invention to provide a sweep integratorsystem in which the distortion ofthe ,video signal is substantiallyindependent of the feedback factor of the recirculating loop.

A further object of the present invention is to provide ,a sweepintegrator system having a greater stability than previously knownsystems.

Another object of the invention is to provide a sweep integrator circuitin which periodic modulation and demodulation of the recirculatingsignal is avoided.V

More particularly, an object of the present invention is to provide asweep integrator system having a stable lfeedback factor greater than0.95. 'Y

Still another object of the invention invention .is to provide a sweepintegrator system in which the stored intelligence is substantiallyindependent of the signal amplitude in the recirculating loop.

These and other objects of the present invention, which willvappear asthe description of the invention proceeds, are generally accomplished ina sweep integrator system employing a frequency-modulated signal in arecirculating loop. Such a frequency-modulated sweep integratorcomprises generally a delay line, a feedback circuit coupling ltheoutput of the delay line to the input thereof with means in the feedbackcircuit for controllably changing the frequency of the recirculatingsignal. In the absence of any signal applied to the frequency changingmeans, the recirculating loop will oscillate at a substantially constantfrequency. The frequency changing means is operative to shift thefrequency of the recirculating signal by an amount proportional to theinstantaneous ampltude of a signal applied thereto. This frequencyshifted signal will travel around the loop and will reappear at thefrequency changing means after an interval equal to the delay time ofthe delay line. vThe recirculating loop has an effective feedback factorof 1 for the stored intelligence since only the amplitude and not'thefrequency of the signal will be altered in passing around the loop. TheAfeedback factor of theover-all integrator system is reduced tio .thedesired .value of .9,8 to .99 by providing lan auxiliary low gain,degenerative loop including Va ,frequency sensitive means forgeneratingiasignal.proportional to the instantaneous frequency of thesignal in the 'recirculating .loop and applying this `'generatedsignal-to the Vfrequency changing means'in a Vmanner to cause a vreduction inthe frequency deviation of the recirculating signal. jBy causing thisauxiliary loop to lhave a very low ga-in, for example 0.01 vto 0.02; theyeffective feedback factorof Athe over-all system will Ibe of the orderof .99 to .98. Y

VFor a better understanding of the Apresent invention reference shouldnowbe made tothe following detailed description which is to be Vreadvinconnection with the laccompanying drawings, in which: Y

Fig. Il is a .blockv diagram of an embodimentk of the present inventionemploying a single frequency changer intherecrculating loop;

Fig. ylA is a plot showing the lgain versus frequency characteristic .ofone elementV of the embodiment of Pig. 2 is Ta block diagram of apreferred embodiment of the invention employing multiple heterodyning inthe recirculating loop;

Fig. 3 is a block diagram of a third embodiment of the present inventionin which the signals applied to the twomixers inthe recirculating loopare .derived from the same oscillator;

"Fig, '4 isan embodiment ofthe present invention in vwhich automatic.frequency control .of vone of the oscillators is provided in thedegenerative feedback loop; i .Fig 4A is a schematic diagram of .afilter network in the degenerative loop; and

jFrig. 4B is a curve showing the frequency response Vcharacteristic ofthe 'Klter network of Fig. 4A.

'ing a delay time equal 'to the repetitionperiod of the As shown in Fig.1A, filter 14 has a maximum gain (lowest attenuation) at a 'frequencyf1. VThe gain of filter 14 drops rather sharply on either side offrequency f1 and then remains substantially constant over a rather widepassband. The characteristic shown in Fig. lA may be obtainedbyrincluding, in a conventional band-pass' filter circuit, one or morehigh Q circuits resonant at the Y frequency f1. For example, theband-pass filter 14 may comprise five amplifier stages which have tunedcoupling circuits therebetween. The desired *characteristic shown inFig. lA can be achieved by stagger tuning these stages in a conventionalmanner and selecting the coupling circuit'of the Ymiddle lstage to havea higher Q than the other four. In general, the ratio of the gain offilter 14 at frequencies slightly displaced from frequency f1 to thegain at frequency f1 should be equal to or greater than the over-allfeedback factor of the system in order that the signal inthereeirculating loop .will stabilize at the frequencyV f1. Ihe outputof amplifier 16 is coupled to the input of delay line 10 through a shorttime constant video limiter 18. The alpha loop will oscillate at asubstantially constant amplitudeat; frequency f1 since the gain aroundthe alpha loop is held -`at unity for this frequency by the action oflimiterl18 and filter 14. The alpha loop will not sustain-continuous,oscillations at frequencies other filter 'V14 at frequencies displacedfrom f1. lTwo gains periodic signal to be integrated*` lf theembodimentof Fig. '1 is to .be used as asweep .integrator in Aa ra'dar system,delay linellf) will havea delaytime equal to the Ypulse repetitionperiod vof the radar system. Coupled to thegoutput of delay line 10 isafrequency modulator 12.

"Frequency modulator 12l may be any of, a number of Vfrom a reference orcarrier Afrequency by a frequency interval that is a function of theamplitude ofthe modulating signal. Frequency modulator 12 preferablyincludes an integrating circuit preceding the modulating 'circuit sothat the displacement ofthe instantaneous frequency from the frequencyof the recirculating signal is proportional to .the amplitude of themodulating signal rather than proportional to the differential o'f themodulating signal as-is the case in conventionalv phase Vmodu- 'latorcircuits. This system is known as the vArmstrong system ofyfrequency'rnodulation,and is more .fully described in 24 Proceedings ofthepI. R. E.` 689, May 1936.

.The output of .frequency ,modulator .12- .is supplied through `aband-pass 'filter 14 to the input of an amplifier 16. Band-passY filter14,has, apassband characteristicas shown in'Frig. lA which isa-plotofgain versus frequency.

must be considered in connection with any consideration of the alphaloop. One ygain is the gain for the intelligence circulatingin the alphaloop. This gain is always equal to unity for the alpha loop `since theintelligence is carried by a shift in'fr'equency of vthe recirculatingsignal and is independent of the amplitudeof the recirculating signal.`The second gain vis the amplitude gain of the recirculatingsignal. Thisgain must have an average value of approximatelyrunity in order that aysignal may be continuously recirculated in' thealpha loop. It is thegain for the intelligence that is reduced by the action of the betaloop.

The output of filter 14 is also coupled toa frequency discriminator 420having a center or zero output frequency f1; -Discriminator i20 iscoupled through a signal attenuator 22 to one'k input of signal addingcircuit` 24. It willbeapparent as Athe description of the inventionproceeds 'thatthe peakin the Vcharacteristic of filter 14 is notessentialto Athe'jproper operationof the present invention since di'scriminatorl also helps to stabilize the frequency circulating in thealpha loop at the frequency f1. Adding Ycircuit'jf24 is coupled tomodulator '.12 and supplies the .modulating signal. thereto. TheVideosignal to be lintegrate'dis applied to .a second input v26 lofaddingcirouit'24. Adding `circuit v24 may take the vform of twovideoamplier stages witha common anode load impedance, Athe controlsignal for one amplifier being supplied frominput '26 land rthe controlsignal for the other amplifier being supplied from attenuator 22.Attenuator 22m-is` provided toreduce thergain of Athe beta loop to the.desired lowvalue. Theoutput signal is derived from the output ofVdiscriminator 20 Vat terminal 2S.

The operation'v of Fig.l l may be' explained as follows. In the absenceofa videosignal at terminal 26, the signal inthe alpha loop .will `be `aconstantamplitude, constant frequency signal. The frequency of -thissignal will be the frequency f1 `of Fig. :lA. If a single video pulse isapplied rat terminali@.modulator .12 willcause the frequency of the.signal in the alpha loop to shift for the 'duration ofthe applied,pulse tbyim amount proportional at modulator 12 ,afterga `time:interval determined bythe delayA-.timeqof 4.the .alphagloop .If4 nootherkpulses --are appliedat terrniual .2ti, this frequency shiftedsignal :will

continue to circulate around the alpha loop with the difference betweenthe frequency of this 'signal and the frequency f1 diminishing slightlyeach time the signal makes one complete circuit around the alpha loop.This decrease in the frequency deviation is caused by the feedbackprovided by the beta loop. When the frequency shifted signal appears inthe output of band-pass filter 14, discriminator 20 will have an output,the polarity of which is dependent upon the direction of the excursionfrom frequency f1 and an amplitude proportional to the amplitude of theexcursion. The output of discriminator 2t) is passed through attenuator22 and applied to adding circuit 24. Attenuator 22 is adjusted so thatthe signal applied to adding circuit 24 will cause a frequency excursionopposite in direction to and from .Ol to .02'times the amplitude of theoriginal frequency excursion. The delay around the beta loop isextremely small so that the degenerative signal provided 'by thebetaloop is applied to modulator 12 at the time that the original frequencyexcursion is passing through modulator 12. Therefore, the action of thebeta loop `will be to cause a small decrease in the original frequencyexcursion each time this frequency shifted signal passes around thealpha loop. Itrshould be noted that this action of the beta loopwillalso tend to stabilize the frequency of the alpha loop 'at 'thefrequency f1. That is, if the frequency of the circulating signal shoulddepart from the center frequency of discriminator 2) for a long intervalrather than momentarily, modulator 12 will respond to the continuoussignal from the beta loop and will shift the frequency of therecirculating signal in the direction of frequency f1.

If the signal applied at terminal 26 is a pulse which recursperiodically at intervals equal to the delay time of delay line 10,adding circuit 24 will supply a signal to modulator 12 tending to causean additional frequency excursion in the recirculating signal each timethe original frequency shifted signal .reappears at modulator 12.` Thenet result will be that a single frequency shifted signal continues tocirculate around the alpha loop but the amplitude of the excursionmeasured from frequency f1 is increased on each trip around the alphaloop by the periodic signal applied at terminal 26 and decreased eachtrip'by the degenerative signal derived from the beta loop. Anequilibrium condition will be reached in which the amplitude of thedegenerative signal supplied by the beta loop is exactly equal to theperiodic signal applied at terminal k26. If the signals applied atterminal 26 vary in amplitude, the amplitude of the frequency excursionin the alpha loop will follow this change in amplitude of the appliedsignal after a slight time delay. Ifthe signal applied at terminal 26 isa periodically recurring series of pulses rather than a periodicallyrecurring single pulse, there will be several frequency shiftedsignals'circulating in the alpha loop at one time. The time intervalsbetween these frequency shifted signals at any point in the alpha loopwill be equal respectively to the time spacings of the correspondingpulses in the signal applied at terminal 26. For example, if the pulseseries consists of three pulses spaced 20 microseconds apart and therepetition period of the pulse series is 1000 microseconds, at any pointin the alpha loop there will be one frequency shifted signal followed bya second signal 20 microseconds later and a third signal still another`20 microseconds later. Then no frequency shifted signal will appear atthis point for 960 microseconds. Aperiodic signals applied at terminal26 will also cause frequency excursions in the alpha loop but thesefrequency excursions will soon die out owing to the fact that they areattenuated by the beta loop without receiving reinforcement from signalsapplied at terminal 26. A's will be seen from the above discussion,adding circuit 24 may be either an adding or a subtracting circuitdepending upon the polarity of the output of discriminator 20. `However,the combined action of discriminator 20 and adding circuit 24 mustbesuch as to cause the-beta loop to supply a degenerativesignal to thealpha loop. VThe embodiment of the invention shown in Fig. 2 operates onthe same general principle as the embodiment of Fig. 1 but differs fromthe embodiment of Fig. 1 in the manner of introducing a frequencyexcursion into the alpha loop. The frequency modulator 12 of`Fig..1 isreplaced in Fig. 2 by a heterodyne mixer 34 and a deviable oscillator36. Adding circuit 24 is coupled to deviable oscillator 36 in a mannerto control the frequency of this oscillator. Oscillator 36 may include areactance tube or any one of a number of other Well known meansresponsive to the signal from adding circuit 24 to deviate the frequencyof the oscillator by an amount proportional to the instantaneousamplitude of the applied signal. Mixer 34 is inserted in the alpha loopbetween delay line 10 and band-pass filter 14. Since the signal fromdeviable oscillator 36 will either raise or lower the frequency of therecirculating signal in the alpha loop even in the absence of a signalat terminal 26, it is necessary to compensate for this change in averagefrequency at some other point in the alpha loop. In Fig. 2, thiscompensation is provided by a mixer 38 inserted in the alpha loopbetween limiter 18 and-delay line l0. A second deviable oscillator 40,having the same average frequency as oscillator 36, is coupled to mixer38. An automatic frequency control (AFC) circuit 42 is coupled to theoutputs of oscillators 36 and 40 Vto insure that the average frequencyof oscillator 40 remains exactly equal to the undeviated frequency ofoscillator 36. The output of automatic frequency control circuit 42 iscoupled back to oscillator 40 to adjust the frequency of this oscillatorto correct for any frequency drift. Automatic frequency control circuitsfor performing the functions outlined above are well known in thefrequency modulation art where they are used to maintain the averagefrequency of the deviated oscillator equal to the frequency of a crystalcontrolled standard frequency.

Band-pass filters 44 and 46 are insertedbefore and after mixer 38,respectively, in order to eliminate undesired sideband or heterodynesignals. Filter 44 has a characteristic similar to filter 14 with orwithout the peak at frequency f1. Filter 46 has a passband equal inwidth to the passband of filters 14 and 44 but centered about afrequency equal to f1 increased or decreased by the frequency ofoscillator 40.

Additional amplifiers (not shown) may be inserted in the alpha loopbefore and/or after delay line 10. If such additional amplifiers areinserted, the gain of amplifier 16 should be decreased in order tomaintain a gain of approximately unity` around the alpha loop. Theaction of limiter 18 will reduce the gain of the loop to unity. In anactual physical embodiment of the invention, filters 14, 44 and 46 mayform the input or output circuits of various amplifiers and mixers inthe alpha loop.

It is believed that the understanding of the invention in general and ofthe embodiment of Fig. 2 in particular will be facilitated by assigningspecific values to the frequencies appearing in the embodiment of Fig.2. These assigned values serve only to illustrate the invention and donot in any way set limits of frequencies within which the invention willoperate. Let it be assumed that the alpha loop has a natural frequencyof oscillation of 7 mc. measured at the input of amplifier 16. In thiscase, band-pass filters 14 and 44 will have a pass-band centered at 7mc. Oscillator 40 may have any suitable frequency of oscillation, forexample 11 mc. The frequency of the signal in the outputv of mixer 38will be the sum of 11 mc. and 7 mc. plus or minus any frequencyexcursion resulting from applied video signals. Therefore, band-passyfilter 46 will have a passband `centered at approximately 18 mc.Oscillator 36 should have an undeviated Yfrequency of approximately11mc. so thatthe difference in fre quency .between ethe.. signal .at thevoutput .of delay .line 10 and the :signal from,.qscillator 3.6. willagain ,Abe .7 pic. The-properffrequencyrelationship between oscillators36 and40 ismaintained by :automatic frequency control cirrCuit 4 2 as.fdeseribed'tabove .fDiscrimiuator 20 has a center. frequency equaltofthenaturalifrequency of the alpha loop, in thisexample .a frequencyof 7 mc. lDiscriminatorv20 ;will act to .hold the oscillation vfrequencyat the assumedfrequency, of 7 mc. Assumenow Vthat the alpha loop ofFig.l .is oscillating .at its naturalfre.- quency of 7 mc. and thatno'inforrnation is stored within the-alpha-loop. Assume `further that apulse having a time duration short..c c mpar;ed to the delay1time.of.delay line 10 Vis appliedat input26 of adding circuitg24. Thesignal from :adding circuit 24 .will cause a shift. in the frequency, of.oscillator :for lthe durationof `the input pulse. VLet.it betassumed.thatthe pulse shifts the frequency of oscillator l36 by 08;mc. to,afrequency Vof 11.08

mc. The.output offmixer341will be `shifted for the dura# tion -of theinputgpulse to.a frequencyof 6.92 mc. This shifted frequencyisfwellwithin. thepass-band .of lters 14 and 44 .so that the ysignal attheshifted frequency is coupled around the alpha loopto mixer 38..y .Thesignal .intheoutput Vofmixer will be equal to 6.9.2 me. plus 'l1 mc. or17.92 mc. for the duration of the. applied pulse. rlibe-:signal in theoutput of delay line 10 will continue at 18 mc. until the signal at thefrequency of 17.92 mc.passes down the delay line and appears at theoutput. If a single pulse is applied at terminal 26, the frequency .ofoscillator 36 Lwill again be at ll mc. whenpthe signal at y17.92 mc.appears in the output of delay .-line.10. The :frequency of .the signalin the output mixer 34 will then be equal to 17.92 minus ll rnc. or 6.92mc. Except for the .action of the beta loop, this signal -.at 16.92 mc.-wonld be continuously recirculated Y around the alphaloop. Tofreducethe feedback factor of the alpha .loop .so that signals are ,stored foronly 50 to 100 trips around the alpha loop, asignalis supplied from theoutput .of bandfpass'lter 14 to discriminator 20. Discriminator 20isloperative to produce. an output pulse in -respon'se .tothe momentaryfrequency excursion of the'signal in the alpha loop. The polarity ofthis pulse in Vthe.output ofqdiscriminator `20 isopposite to that of thepulse originally applied at'input 26. Thisv pulse from discriminator 20is attenuated in attenuator 22 to an amplitude approximately 17%@ theamplitude of the original pulse applied atinput'26. .The'application ofthe attenuated signaLto-adding circuit 24 causes the frequencyofoscllator 36 to shift to .a .vfrequencybelow l'l rnc.

by .an .amount .equal to the original shift 'of .08

divided ,by the factor '50 .or .0016 mc. Therefore the frequency ofoscillator 36 willbeapproximately 10.9984 mc. 'This decrease -inyfrequency of oscillator 36 will result ina corresponding'increasevinthe signal at filter 14 from 6.92 mc. 106.9216 mc. Asthefrequencyshifted signal representing the original pulse continues toYrecirculate inV thefalphailoop, the `deviation from frequency f1 willgradually d ecrease until there is no detectablefre,- quency shifted.signal stored in theloop.

Assume now -that the initial pulse has been applied a input 26 and thatthefrequency shifted .signal'has passed around the loop andhas appearedat/the output of delay` line 10. If a second pulse, equal in amplitudeto the first pulse, Visapplied at input 26, this second pulse will causethe frequencyof oscillator 36 to again increase-.to a 'frequencyofapproximately. 171.08 yfmc. Thelsignal vin theoutput of mixer171willfnow-be the difference between .17.92 mc...and M .OS'Tmcjor 6.84mc. This frequency .shifted signal which has a time duration equalto-the time duration ofthe input pulses will again pass aroundftherecirculating loop and .appear in the output o f delay line 10 `asafrequencyof l7. 8.4'rnc. If athird pulse, s tilLof they same amplitude,is yapplied at input 26, the frequency-in -t-he ,output l of -mixer34will( decrease vby @additional .0.8 10,676 -mC-; YThe .actionof theexample but the-reduction will beta loop willreduce slightly thedeviations giveninftbis be comparatively small foronly three pulses. Y

It can be shown mathematically -that the `maximum decrease in frequencyin the output off'mixer'34 vforni series of pulses of uniformamplitudeapplied at input Y26 will be' equal toapproximately Y where-F is theeffective feedback factor around the alpha loop, which in this case isequal to the feedback factor of the alpha loop minus' the feedbackfactor of the beta( loop. If F=. 98 and if a single pulse producesadeviaf 'tion of .08, the maximum deviation in frequency-,at the outputVof mixer 34 will be 4 mc. measured from the natural frequency of'7 mc.1.*

The video output signal at terminal 28 is derived `from the output ofdiscriminator 20 and will have an amplitude proportional to the maximumfrequency deviation in the output of mixer 34. The amplitude of therandom noise and clutter signalwill be greater at the output of:discriminator 20 than at input 26 but by a factor much less than 5.0.vTherefore, the signal-to-noise and/or clutter ratio in the output of thesweep integrator will be higher thanthe signaltonoise and/or clutterratio at input 26 by asmuch as l0 to 2 0 db. j

As suggested above, various modifications may be made in the circuit ofFig. v2 without altering the basic principles of operation of thecircuit. For example,l one or more of band-pass `filters 14, 44 and 46maybe omitted. Furthermore, amplifiers may be included before and afterdelayline 10.in order to obtainthe desired signallevels at these. pointsin the alpha loop. It will be remembered that the amplitude of thesignal in-'the alpha loop .is ynota critical factor sincethe'information is stored .as a change in frequency of the recirculatingsignal rather than as a change in amplitude.,l Attennator.

oscillator byV anamount proportional to theamplitude of thesignalapplied to modulator 62, from adding circuit 24. The signal from addingcircuit 24 may b e passed through an integrator circuit (not shown)before itis applied to lphase modulator 62, so that the deviated fre.-quencies generated in modulator 62 are displacedfrorn the frequency of.oscillator 60 by ya frequency interval directlyproportional to theamplitude of the signalinthe output of adding Vcircuit 24.

The embodiment of the invention illustrated in Fig. 1 4 again employsthe vsame elements in the alpha'loop as doesthelembodiment of Fig. 2.Howeven the betaloop of Fig. 4 includes a Vfilter 70 having a gainversus vfrequencyy characteristic as shown by theV curve 7,2 in Eig, 4B.f Filter 70 replaces attenuator 22 since the attenuaf tion vof filter atfrequencies above some low value, for example l0 cycles p er second, is.equivalent to` the attenuation vprovided by attenuator 22..',Theattenuationv Lof filter 70 at frequencies below l0 cycles is verylowcompared to the attenuation at frequenciesl above l0v cycles persecond. Therefore, a very large feedback signalis supplied to addingcircuit 24 for frequencies from 'zero to l0 cycles per second.l Any slowshift in the frequency of deviable oscillator 74 such as .would occurwith `thermal drift will yresult in a relatively large Ysignal fbeingappliedto adding circuit 24 in a direction to equalize the affecting theoperation of the sweep integrator.

average frequency of deviable oscillator 74 exactly equal to thefrequency of oscillator 76. In the paragraph above it has been assumedthat the frequency of the alpha loop remained exactly equal to thecenter frequency of discriminator 20. If the assumption is now made thatthe oscillators 76 and 74 are operating at exactly the same frequencybut that the natural frequency of the alpha loop has drifted from thecenter frequency of discriminator 20, it will be seen that the frequencyof operation of oscillator 74 will be shifted slightly to restore thefrequency of the signal at the output of filter 14 to the centerfrequency of the discriminator. Therefore the beta loop acts to maintainthe circulating signal at the proper value despite changes in thenatural frequency of the alpha loop and/or changes in the frequency ofoperation of oscillators 74 and 76. The shape' of the characteristic ofthe filter 70 at low frequencies will have very little effect on thevideo signals in the beta loop since the amplitude of such low frequencycomponents-in the video signal is extremely small.

Fig. 4A illustrates a filter circuit having the gain versus frequencycharacteristic shown in Fig. 4B. In Fig.l 4A, capacitors 8f) and 84 lmaybe the distributed capacitances of the circuit elements. The capacitanceof capacitor 84 should be approximately 200 times the capacitance ofcapacitor 80. Series resistor 86 is selected to have a resistance equalto approximately 200 times the resistance of resistor S8. Capacitor 90is chosen so that a frequency response drops off as shown in Fig. 4B.

In the four embodiments of the invention described above the modulationand demodulation processes are confined to the beta loop. Since the gainof the beta loop is always small and degenerative, the distortion andnonlinearities introduced into the integrator system by the modulatorand demodulator circuits are essentially the same as would be obtainedby passing the input directly through these two elements once withoutany feedback. In addition, the distortion in the output does not tend toincrease as the number of recirculations of the information in the alphaloop is raised. In the frequency-modulated integrators described above,the gain of the video signal in the alpha loop does not tend to varywith varying parameters since the intelligence is stored in the alphaloop as a change in frequency. For the same reason, the amplitude of thesignal in the alpha loop may vary within reasonable limits withoutadversely More specifically, the gain tolerances of the alpha loop areno more severe than are those in a standard video amplifier. This is tobe contrasted with the situation which prevails in amplitude-modulatedsweep integrators where loop gain changes of only a few percent maycause an amplitude-modulated sweep integrator to oscillate and becomeuseless.

The invention is not limited in its application to sweep integratorsystems for use in conjunction with radar detection systems. The presentinvention will nd application wherever it is desired to detect aperiodically recurring signal in the presence of a random signal havingan amplitude equal to or greater than the amplitude of the periodicsignal. In many cases the-embodiments described above may beemployedwithout modification to integrate signals derived from systemsother than a radar system. In other instances any modification of theinput circuit that is` required to adapt the embodiments described aboveto such differing applications will be obvious to one skilled in theart. `It can be shown that the over-all gain versus frequencycharacteristic of each of the systems described above is identical tothe gain versus frequency characteristic of a comb filter. That is, thegain of the system is high for signals having a repetition period equalto the delay time of the delay line and for harmonics of such signals.'Ihe gain is low for frequenciesv other than those mentioned above.It'can be shown further that the gain versus frequency characteiistic ofthe filter may be altered to cause the gain peaks to occur at oddmultiples of a signal having a period equal to one-half the delay timeof the delay line by causing mixers 38 and 34 in Fig. 2, for example, toshift the frequency of the signals passing therethrough in the samedirection. That is, if the signal in the output of mixer 38 has afrequency equal to the sum of the frequencies of the signals fromoscillator 40 and band-pass filter 44, then the signal in the output ofmixers 34 will have a frequency equal to the sum of the frequencies ofthe signals from oscillator 36 and delay line 10. It will be rememberedthat in the embodiment described above, the signal in the output ofmixer 34 had a frequency equal to the difference of the frequencies ofthe applied signals. This modification of the system of Fig. 2 willresult in ashift in the sign of the deviation of the recirculatingsignal from the natural frequency of oscillation of the system each timethe signal makes a trip around the loop. Otherwise the operation of themodified system is much the same as that of the system of Fig. 2.Therefore, while there have been described what are presently consideredto be preferred embodiments of the present invention, the scope of thepresent invention is to be determined solely by the limits establishedby the hereinafter appended claims.

We claim:

l. A sweep integrator system comprising a recirculating signal loopincluding a delay means Vand having'an over-all gain of unity, saidsystem including means'for causing a signal at a substantially constantfrequency to be circulated in said loop, means for causing a shift inthe frequency of said recirculating signal dependent upon aninstantaneous characteristic of the signal to be integrated, adegenerative auxiliary loop coupled to said recirculating loop, saiddegenerative loop causing the effective intelligence feedback factor ofsaid recirculating signal loop to have a value less than unity, andmeans for deriving an output signal from said recirculating signal loop.

2. A sweep integrator system comprising a recirculating signal loop,said loop including a delay means for causing signals circulating insaid loop to have a predetermined transit time around said loop, saidloop including means for causing a signal at a substantially constantfrequency to be circulated in said loop, means for causing a shift inthe frequency of said circulating signal proportional to theinstantaneous amplitude of an applied signal, means coupled to said loopfor generating a signal having an instantaneous amplitude proportionalto the instantaneous frequency of said recirculating signal at the pointof coupling, signal combining means coupled to said signal generatingmeans and thesource of said signals to be integrated and means couplingthe output of said signal combining means to said frequency shiftingmeans to control the operation of the latter. Y

3. A system for integrating a periodically recurrent signal, saidintegrating system comprising delay means having a delay time equal tothe period of said signal, a feedback circuit coupling the output ofsaid delay means to the input thereof, thereby to form a recirculatingsignal loop, means forming a part of said feedback circuit for normallymaintaining an approximately constant amplitude recirculating signal insaid loop, said signal normally having a predetermined fixed frequency,frequency changing means also forming a part of said feedback circuit,said frequency changing means being operative to cause the frequency ata point in said loop to shift by an amount proportional to theinstantaneous amplitude of a signal applied thereto, means coupled tosaid loop for generating an output signal proportional to theinstantaneous deviation from said predetermined frequency of thefrequencyat the point of coupling to said loop, means for additivelycombining the output signal of said' last-mentioned signal generatingmeans and said signal to be integrated, said signal 'combiningmeansbeing coupled 11 Y to ,saidfrequency changing means to control thefrequencyrthereof. f -V f -4. lA system for integratinga periodicallyrecurrent signal, said integrating system comprising delay means havingla delay time equal to the period ofvsaid signal, afeedback circuitcoupling the output of said delay means lto the input thereof, therebyto `form a recirculating signal loop, means forming a part of saidfeedback circuit-for` normally maintaining an approximately constantamplitude recirculating signal in said loop, said signal normally havinga predetermined fixed frequency, frequency lchangingv means alsoyforming a part of said loop, Vsaid'frequency changing means beingoperative to cause the frequency at a point in said loop to shift by anamount proportional to the instantaneous amplitude of a-signal appliedthereto, meanscoupled to said loop for generating anV output signalproportional to the instan-y taneous deviation from said predeterminedfrequency of thev frequency -at'the point of coupling to said loop, asignal adding circuit,` means coupling the output signal from saidlast-mentioned signal generating means toV a first input ofsa'id'signalVadding circuit, means for coupling said signal to be integrated to asecond input of said signal adding circuit, and means coupling theoutput of said signal adding circuit to the input of said frequencychanging means. Y f

' '5.- A system for integrating periodically recurrent pulse series,said integrating system comprising delay means t having a time delayequal to the timeV spacing between corresponding pulses in successivepulse series, a feedback circuit coupling the output of said delay meansto the input thereof, thereby to form a'recirculating signal loop,meansv forming a part of said feedback loop for normally maintaining anapproximately constant amplitude recirculating signal in said loop,frequency sensitivey means associated with said loop and constructed andarranged to cause said signal normally to have a fixed predeterminedfrequency, the period of one cycle of said signal being short comparedto the delay time of said delay means, pulse operated frequency changingmeans also forming a part of said signal loop, said frequency changingmeans being operative to cause the frequency at a point in said loop toshift by an amount proportional to the amplitude of a pulse applied tosaid frequency changing means, the duration of said frequency shift atsaid point being equal to the duration of said pulse, frequencysensitive means coupled to said loop',-said frequency sensitive meansbeing operative'to generate an output signal proportional to theinstantaneous deviation from said fixed frequency of the frequency atthe point of couplingto said'loop, a signal adding circuit7 meanscoupling the output of said frequency sensitive means to a first inputof said signal adding circuit,'means for supplying the signals to beintegrated to a second input of said signal adding circuit, and meansfor supplying output from said signal adding circuit to the input ofsaid frequency changing means.

6. A system for integrating periodically recurrent signals, saidintegrating system comprising a delay means having a delay time whichbears a predetermined relationship to the period of said signals, afrequency changing means, a band-pass filter having a lesser attenuationat one frequency Within the passband thereof than at other Vfrequencieswithin said passband, and an amplifier, the above-named elements beingconnected in a series loop, said series loop-being operative to generatea recirculating, oscillatory signal of approximately constant amplitudeat the frequency of least attenuation of said filter, the period of saidsignal being short compared to the delay time of said delay line, meansfor supplying the signal to be integrated to said frequency changingmeans-to control the operation thereof, `said series loop having afeedback factor ofunity for, information stored s `a shift in frequencyof said recirculating signal, means coupled to saidlo'op f'orgeneratingan output signal pro- 12 portional irl-amplitude to the instantaneousdeviation from saidjfrequency of Vleast attenuation of the frequency atthe point of coupling to said loop, and means for supplying a portion ofsaid output signal degeneratively to said series loop, thereby to reducethe effective feedback factor ofsaidloop to a value lessthan unity.

7. A system .for integrating periodically recurrent signals, saidintegrating system comprising a delay means having a delay timewhichbears a predetermined relationship tothe period of said signals, afrequency changing means, a band-pass filter having a lesserattenuation` at one frequency within the passband thereof than at otherfrequencies within said passband, and an amplifier, the above-namedelements being connected in a series loop, saidseries loop `beingoperative to generate a recirculating oscillatory signalof approximatelyconstant amplitude at the frequency of least attenuation of said filter,the period ofsaid signal being short compared to the delay time of saiddelaisl line,'meanscoupled to said loop Vfor Vgenerating an outputsignal proportional in amplitude to the -instantaneous deviation fromsaid frequency of least attenuation of the frequency atthe point ofcoupling to said loop, a signal adding circuit, means for Vsupplyingoutput signal from said'last-'rrlentioned signal generating means to afirst input of saidsignal adding circuit, means for supplying 'saidsignals to be integrated to a second input of said signal addingcircuit, and means for VKsupplying output from'said signal addingcircuit to the input of said frequerrcy changing means, said frequencychanging means beingoperativeV to cause the frequency of a signalpassing therethrough to shift by -an amount proportional toV theinstantaneousI amplitude of a signal applied thereto.

v8. A signal integrating vsystem las in claim 7 wherein saidsignalsupplied'to said first input of said signal adding circuit causes saidfrequency changing means to shift said frequency passing therethrough ina direction to reduce said deviation rof said frequency at the point ofcoupling to said loop, said reduction in deviation being small cornparedto the total deviation in frequency of said signal at said point ofcoupling l 9. A system forintegrating a composite signal which includesa periodically vrecurrent series of pulses, said integrating systemcomprising a delay means having aY delay time Vequal to the timeinterval between corresponding pulses in successive pulse series,frequency changing means, a band-pass filter having a frequency of leastattenuationapproximately in the center of the passband thereof, anamplifiery a limiter, the above-mentionedelements being connected inaseries loop adapted to generate a recirculating loscillatory signal Vofsubstantially constant-amplitude at said frequency of least attenuationof said filter, the period of said signal-being shortcompared to thedelay' time of said delay means, a discriminator circuit coupled to saidloop for generating a signal proportional to the instantaneous deviationfrom said frequency-of -least attenuation of the frequency at thepointhof' coupling to said loop, a signal adding circuit,

o means for supplying output from said discriminator to a first input ofsaid 'signal adding circuit, means for supplying theosignal to' beintegrated to a second input of said signal adding circuit, and meansfor supplying output from said signal ac'iding-cincuitV to the input ofsaid frequency changing means, said frequency changing means beingadapted to'cause the frequency of a signal passing therethrough to shift`byv an amount proportional to the' ydelay time equal Vto the intervalbetween successive pulse series, a band-pass'ifilter, an amplifier, alimiter circuit,

Vand a frequency modulator, the aforementioned elements being connectedin a series loop, said series loop having a substantially constantnatural frequency of oscillation, the period of said oscillation beingshort compared to the delay time of said delay line, a discriminatorcircuit coupled to said loop at a point following said band-pass filter,said discriminator circuit being operative to generate a signalproportional in amplitude to the instantaneous deviation from saidnatural frequency of oscillation of the frequency at the point ofcoupling to saidV loop, means for additively combining the output ofsaid discriminator and the signal to be integrated, means forsupplyingoutput from said signal combining means to the input of said frequencymodulator to control the operation thereof, said discriminator and saidsignal combining means forming a low gain degenerative feedback looptending to reduce the frequency deviation at said point of coupling tosaid loop, and means for deriving an integrated signal from the outputof said discriminator circuit.

ll. A system for integrating a composite video signal which includes aperiodically recurrent series of pulses, said system comprising a first'mixer, a first band-pass filter, an amplifier, a second mixer, a`secondband-pass filter and a delay line, the aforementioned elements beingcoupled in a closed series loop in the order recited, said series loophaving a substantially constant natural frequency of oscillation, theperiod of said oscillation being short compared to the delay time ofsaid delay line, a first source of oscillatory signals coupled to saidsecond mixer, said second mixer being operative to generate a beatsignal having an instantaneous frequency vequal to the sum ofthefrequency of the signal from said first source and the instantaneousfrequency of the signal in the output of said amplifier, the frequencyof said beat signal falling within the passband of said second bandpassfilter, a second source of oscillatory signals coupled to said firstmixer, said second mixer being operative to generate a second beatsignal having an instantaneous frequency equal to the difference betweenthe instantaneous frequency of the signal in the output of said delayline and the frequency of said second source, said second beat frequencylying within the passband of said first band-pass filter, said secondsource having an undeviated frequency equal' to the frequency ofsaidfirst source, a discriminator circuit coupled to the output of saidfirst band-pass filter, said discriminator circuit being operative togenerate a signal proportional in amplitude to the instantaneousdeviation from said natural frequency of oscillation of the frequency ofthe signal in the output of said second filter, means for additivelycombining the output of said discriminator and said signal to beintegrated, means for supplying output from said signal combining meansto said'second source to control the frequency generated thereby; saiddiscriminator and said signal combining means forming a degenerativefeedback loop having a gain less than unity.

l2. Anintegrating system as in claim ll wherein said second source ofoscillatory signals comprises afrequency 'modulator coupled to`said`first source of oscillatory signals. l

13. A system for integrating a composite video signal which includes aperiodically recurrent series of pulses, said system comprising a firstmixer, a first band-pass filter, an amplifier, a second mixer, a secondband-pass filter and a delay line, the aforementioned elements beingcoupled in a closed series loop in the order recited, said series loophaving a substantially constant natural frequency of oscillation, theperiod of said oscillation being short compared to the delay time ofsaid delay line, a first oscillator coupled to said second mixer, saidsecond mixer being operative to generate a beat signal having aninstantaneous frequency equal to the sum of the frequency of the signalfrom said first oscillator and the instantaneous frequency of the signalin the output of said amplifier, the frequency of said beat signalfalling within the passband of said second band-pass filter, a

mixer being operative to generate a second beat signal having aninstantaneous frequency equal to the difference between theinstantaneous frequency of the signal in the output of said delay lineand the frequency of said deviable oscillator, said second beatfrequency lying within the passband of said first band-pass filter, saiddeviable oscillator having an undeviated frequency equal to thefrequency of said first oscillator, a discriminator circuit coupled tothe output of said first band-pass filter, said discriminator circuitbeing operative to generate a signal proportional in amplitude to theinstantaneous deviation from said natural frequency of oscillation ofthe frequency of the signal in the output of said second filter, asignal adding circuit, means for supplying output from saiddiscriminator to a first input of said signal adding circuit, means forsupplying said signal to be integrated to a second input of said signaladding circuit, means for supplying output from said signal addingcircuit to said deviable oscillator to control the frequency thereof,said discriminator and said signal adding circuit forming a degenerativefeedback loop having a gain less than unity.

14. An integrating system as in claim 13, said system further comprisingfrequency control meansA coupled to said first and second oscillatorsfor maintaining the frequency of said first oscillator substantiallyequal to the undeviated frequency of said second oscillator.

15. A system forintegrating a signal which includes a periodicallyrecurrent series of pulses, said system comprising a delay line having adelay time equal to the time interval between corresponding pulses insuccessive puls'e series, a feedback circuit coupling the output of saiddelay means to the input thereof, thereby to form a recirculating signalloop, means forming a part of said feedback loop for normallymaintai-ning an approximately' constant amplitude recirculating signalin said loop, said signal normally having predetermined,l fixedfrequencies at various points in said loop, first and second signalmixers forming a part of said feed-back loop, said first signal mixerpreceding `and said second signal mixer following Isaid delay line insaid loop, an oscillator coupled to said first mixer, said first mixerbeing operative to alter the instantaneous frequency of therecirculating signal in said loop by the frequency of said rstoscillator, signal generating means coupled to said second mixer, saidsecond mixer being operative to alter the instantaneous frequency of therecirculating signal in Ia direction opposite to said first alterationin frequency and by an vamount 'equal to the `frequency of the signalfrom said generating means, said signal generating means being operativeto generate a signal deviating in frequencyffrom the frequency of saidoscillator by an amount proportional to the amplitude of a signalapplied thereto, frequency sensitive means coupled to said loop at apoint following said second mixer and precedingsaid first mixer, saidfrequency sensitive means being operative to produce 'an output signalproportional in amplitude to the instantaneous deviation from saidpredetermined frequency of the frequency at the point of coupling, meansfor combining the output of said frequency sensitive means and 'saidsignal to be integrated, means for supplying said combined `signal tosaid signal generating means to control the output frequency thereof,and means for deriving an -output signal from the output of saidfrequency sensitive means'.

16. A system for integrating a signal which includes a periodicallyrecurrent series of pulses, said system comprising a delay line having adelay time equal to the time interval between corresponding pulses insuccessive pulse series, a feedback circuit coupling the output ofsaiddelay line to the input thereof, thereby to form a recirculatingsig-nal loop, means forming a part of said feedback loop for normallymaintaining an approximately constant amplitude recirculating signal insaid loop, said signal normally having predetermined fixed frequenciesat'various points in said loop, first and second signal mixers forming apartof said feedback loop, said first signal mixer preceding said -delayline and said second signal mixer following said delay line in saidloop, a first oscillator coupled to said first mixer, said first mixerbeing operative to alter the instantaneous 'frequency of therecirculating signal in said loop by'frequency of said first oscillator,a deviable -oscillator coupled to said second mixer, said second mixerbeing operative toY alter the instantaneous frequencyv of'therecirculating signal in a direction opposite to said first alteration infrequency and by an amount-equal tothe frequency of said deviableoscillator, frequency sensitive means coupled to said lop at a pointfollowing said second mixer and preceding said first mixer,Vsaid'frequency sensitive means being operative to produce an outputsignal proportional -in amplitude to the instantaneous deviation fromsaid predetermined frequency of the frequency at the point of coupling,means for combining the output of said frequency sensitive means andsaid signal to be integrated, means coupling said signal combining meansto said deviable oscillator to control the frequency Vthereof, and meansfor deriving an output signal'from-said frequency sensitive means.- v

17. lA system for intgeratinga -signal which includes a periodicallyrecurrent series of pulses, said system comprising a delay line having adelay time equal to the time interval between corresponding pulses insuccessive pulse series, a feedback circuit'coupling Ythe output lofsaid delay line to the input thereof, thereby to form a recirculatingsignal loop, means forming a part of said feedback loop for normallymaintaining an approximately constant amplitude recirculating signal Yinsaid loop, said signal normally having predetermined fixed frequenciesat various points insaid loop, first and sec-ond ysignal mixers forminga part of said feedbackV loop,'said first signal mixer preceding saiddelay line and said second Vsignal mixer following said delay line insaid loop, a first oscillator coupled to said first mixer, said- `firstmixer being roperative to alter-the instantaneous frequency of therecirculating signal -in said loop by frequency of said firstoscillator, a deviable oscillator coupled to said second mixer, saidsecond mixer being operative to alter the instantaneous frequency of therecirculating signal in a direction opposite to ysaid first alterationin frequency and by an amount equal to the frequency of said deviableoscillator, a discriminator coupled to said loop at a point followingsaid second mixer and preceding said first mixer, a filter circuitc-oupled to the output of said discrimantor, lsaid filter having -arelatively constant, low gain at frequencies above a, selectedrelatively low frequency and a relatively high gain at frequencies belowsaid selected frequency, means for additively combining the output oflsaid filter circuit and said signal to be integrated, said signalcombining means being coupled to said deviable oscillator to con- Vtrolthe frequency jthereof, yand means for deriving an output Isignal fromsaid discriminator. v

18. A system for integrating periodically recurrent signals-eomprisingaseries loop including a delay means through which is passed `a modulatedVcarrier signal, means-for recirculating .said modulated carrier .signalthrough the .delay means and means for modifying said modulated carrierin accordance with the signal to'be integrated, degenerative meansassociated with said loop for causing the intelligence feedback factorof said loop to have a value lessV than unity and means associated withsaid series .loop Vfor providing an output signalrelated to 'themodulation of said carrier signal.

19. A filter for passing input signals of kone characteristic more'readily than Vthose of another characteristie, said filtercomprising asignal delay means, means for modulating said! input signals upon acarrier signal, means for causing lsaid modulated carrier signal to becontinuously recirculated through said delay means, means for sensingthe'modulation of said modulated carrier signal to produce a detected.signal and means for 'combining said detected signal kand saidrecirculating modulated carrier signal.

20. A sweep integrator system comprising a recirculating signal loop,said loop including a .delay means for causing signals .circulating Vinsaid loop to have a vpredetermined transit time around said loop, .meansassociated with said loop .for causing :a 'signal at a substantiallyconstant frequency to .be circulated in 'said loop, means for causing ashift in frequency of said circulating signal'proportional totheinstantaneous yamplitude of an applied signal, means coupled to saidloop for generating a signal :having an instantaneous 'amplitudeproportional to .the instantaneous frequency of said recirculatingsignal at the point of coupling, .signal .combining means coupled tosaid signal generating means and the source of said signals to beintegrated and means coupling the output of said signal combining meansto said frequency shifting imeans to control the operation of thelatter.

21. A filter for vpassing input signals of one characteristic. morereadily than those Vof another characteristic, said lter comprising asignal delay means, means for frequency'modulating afcarriersignalbysaid input signals, means for causing said modulated carrier signal tobe continuously Vrecirculated through said delay means, meansY forlsensing the frequency modulation of said modulated carrier signal toproduce a detected signal, and

means responsive to said detected signal for decreasingl theffrequencymodulation of said modulated carrier signal.

Y. References Cited in the file of this patent UNITED STATES PATENTS`2,426,187 `Earp Aug. 26, 1947 2,446,244 Richmond Aug. 3, 1948 2,487,995Tucker Nov. v15, 1949 2,532,546 Forbes Dec. 5, 1950

